Recently, with an increase in the use of various kinds of electric facilities and applied electronic facilities, an EMI (Electro-Magnetic Interference) is in a rapid increase. It is pointed out that the EMI causes, in addition to causing malfunction and disturbance of the electric and electronic facilities, health problem to operators of the facilities. Accordingly, in the electronic and electric facilities, it is demanded to suppress the intensity of the electromagnetic wave emission within a standard or restriction.
As a countermeasure against the EMI, the electromagnetic wave has to be shielded. In this connection, it is obvious that the property of metal which does not transmit the electromagnetic wave may well be utilized. For instance, a method of forming a chassis with a metal or a highly conductive material, a method of inserting a metal plate between circuit boards, and a method of covering a cable with a metal foil are adopted. However, in the CRT and PDP, since an operator has to recognize characters displayed on a screen, the transparency in the display is necessary. Accordingly, since, in all of the above-mentioned methods, in many cases, front surfaces of the displays are obscured, these are improper as a method of shielding against electromagnetic wave.
In particular, since the PDP generates a lot of electromagnetic waves in comparison with the CRT, stronger electromagnetic wave shielding capability is necessary. The electromagnetic wave shielding capability can be conveniently expressed with a surface resistance value. While, in the light-transmitting material shielding electromagnetic wave for CRTs, a surface resistance value of substantially 300 Ω/sq or less is demanded, in the light-transmitting material shielding electromagnetic wave for PDPs, 2.5 Ω/sq or less is demanded and, in a commercial plasma TV with a PDP, it is highly necessary to be 1.5 Ω/sq or less, and more desirably such high electrical conductivity as 0.1 Ω/sq or less is demanded.
Furthermore, a demand level of the transparency is substantially 70% or more for the CRTs and 80% or more for the PDPs, and further higher transparency is desired for the PDPs.
In order to overcome the above problems, as shown below, various materials and methods that make use of a metal mesh having an aperture and thereby combine the electromagnetic wave shielding capability and the transparency have been proposed.    (1) Conductive Fiber
For instance, in JP-A-5-327274 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), a material shielding electromagnetic wave made of a conductive fiber is disclosed. However, there is a disadvantage in that since the shielding material is large in a mesh line width, when it is used to shield a display screen, a screen becomes darker and characters displayed on the screen are difficult to recognize.    (2) Electroless-plating Processed Mesh
A method where an electroless-plating catalyst is printed in a grid pattern by a printing method and then the electroless-plating is applied is disclosed in JP-A-11-170420 and JP-A-5-283889. However, since a line width of the printed catalyst is such large as substantially 60 μm, the method is not suitable for use in a display that requires a relatively small line width and a dense pattern.
Furthermore, a method where a photoresist containing an electroless-plating catalyst is coated, followed by exposing and developing to form a pattern of the electroless-plating catalyst, further followed by performing electroless-plating is disclosed in JP-A-11-170421. However, the visible light-transmittance of the conductive film, being 72%, is insufficient in the transparency. In addition, since extremely expensive palladium must be used as the electroless plating catalyst, most part of which is removed after the exposure, this method also suffers from a problem of production cost.    (3) Mesh Etched by Use of Photolithography
A method where a thin film metal mesh is formed on a transparent substrate by use of an etching method that makes use of photolithography is disclosed in JP-A-2003-046293, JP-A-2003-023290, JP-A-5-016281 and JP-A-10-338848. Since this method enables to finely process, there are advantages in that a mesh having a high aperture ratio (high transmittance) can be formed and a strong electromagnetic wave emission as well can be shielded. However, this method suffers from problems of a troublesome and complicated production process and thus high production cost. Furthermore, it is known that there is a problem in that, since the etching process is used, the width of the intersectional points in the grid pattern becomes thicker than that of the straight-line portions. Furthermore, the problem of moire is also pointed out; accordingly, an improvement in the problem is demanded.    (4) Method of Forming Conductive Metallic Silver Pattern Using Silver Salt
A photosensitive material that uses a silver salt has been conventionally utilized mainly as a material for recording and transmitting images and pictures. For example, they are commonly used for photographic films such as negative color films, monochrome negative films, films for movies, color reversal films and so on, photographic printing paper such as color paper and monochrome printing paper and so forth, and an emulsion mask (photomask) and so forth utilizing the fact that a metallic silver pattern can be formed as an exposure pattern. In all of these, an image per se obtained by exposing and developing a silver salt has a value, and the image itself is utilized.
However, because developed silver obtained from a silver salt is metallic silver, it is considered that the electric conductivity of the metallic silver may be utilized depending on the production method. Several proposals utilizing such a principle have been known from old days, and examples of disclosing specific methods of forming a conductive silver thin film include the followings. In the 1960s, a method of forming a metallic silver thin film pattern by use of a silver salt diffusion transfer process, in which silver is deposited on physical development nuclei, was disclosed in JP-B-42-023746 (the term “JP-B” as used herein means an “examined Japanese patent application”). It is also disclosed in JP-B-43-012862 that a uniform silver thin film prepared by utilizing a similar silver salt diffusion transfer process and not showing light transmission has a function of attenuating microwaves. Furthermore, a method of utilizing the above principle as it is to conveniently perform exposing and developing with an instant monochrome slide film and thereby form a conductive pattern is disclosed in “Analytical Chemistry”, Vol. 72, 645 (2000) and WO 01/51276. Still furthermore, a method where, based on the principle of silver salt diffusion transfer process, a conductive silver film that can be used for display electrodes of plasma displays is formed is disclosed in JP-A-2000-149773.
In the methods described in the literatures mentioned above, specially prepared physical development nuclei are uniformly provided irrespective of on an exposed portion and on a non-exposed portion. Accordingly, there is a problem in that opaque physical development nuclei remain on an exposed portion where a metallic silver film is not formed, and thereby light transmission is degraded. The aforementioned problem is serious particularly in a case when the metal pattern material is used as a light-transmitting material shielding electromagnetic wave for a display such as a CRT and PDP.
Furthermore, it is also difficult to obtain high electric conductivity by use of those methods, and attempts of obtaining a thick silver film for higher electric conductivity are accompanied by a problem of degradation of the transparency. Accordingly, even when the aforementioned silver salt diffusion transfer process is used as it is, the light-transmitting material shielding electromagnetic wave excellent in the light transmission and electric conductivity and thus suitable for shielding electromagnetic waves from an image display screen of an electronic display facility cannot be obtained.
Still furthermore, when, without using the silver salt diffusion transfer process, the electric conductivity is imparted by using a commercially available usual negative film and by subjecting to development, physical development and plating processes, obtainable conductivity and optical transparency are insufficient for use as a light-transmitting material shielding electromagnetic wave for CRTs or PDPs.
Furthermore, in JP-A-2004-221564, a method of manufacturing a light-transmitting material shielding electromagnetic wave that applies a silver salt photosensitive material is disclosed. In the manufacturing method according to related art described in JP-A-2004-221564, a silver salt-containing layer disposed on a support is exposed and developed, followed by applying a physical development and/or plating process to a developed metallic silver portion to allow carrying conductive metal particle. According to the method, a light-transmitting film shielding electromagnetic wave that has high EMI shield property and optical transparency can be obtained.
As mentioned above, when a light-transmitting material shielding electromagnetic wave is formed by use of a silver salt photosensitive material, silver formed by developing (developed silver) is desirably made smaller in the electric resistance. When the resistance of the developed silver is more lowered, the shielding property of the light-transmitting material shielding electromagnetic wave can be improved and thereby its utility value can be heightened. Furthermore, when the physical development and/or plating process is applied to the developed silver like in JP-A-2004-221564, when the electric resistance of the developed silver is made smaller, an improvement in the plating velocity or an improvement in the uniformity can be expected.
In order to further reduce an electric resistance value of the developed silver, it is considered effective to reduce a binder such as gelatin from a silver salt photosensitive material to increase the density of silver halide particles. However, since there is a limit, a technology of further reducing the electric resistance value of the developed silver is in demand.